1. Field of the Invention
This invention relates generally to surface-emitting lasers and, in particular, to removal of heat generated in the active region of vertical-cavity surface-emitting lasers (VCSELs).
2. Description of the Related Art
The following descriptions and examples are not admitted to be prior art by virtue of their inclusion within this section.
Lasers have a wide range of industrial and scientific uses. There are several types of lasers, including gas lasers, solid-state lasers, liquid (dye) lasers, and free electron lasers. Semiconductor lasers are also in use. The possibility of amplification of electromagnetic waves in a semiconductor superlattice structure, i.e., the possibility of semiconductor diode lasers, was predicted in a seminal paper by R. F. Kazarinov, et al., xe2x80x9cPossibility of the Amplification of Electromagnetic Waves in a Semiconductor with a Superlattice,xe2x80x9d Soviet Physics Semiconductors, vol. 5, No. 4, pp. 707-709 (October 1971). Semiconductor laser technology has continued to develop since this discovery.
There are a variety of types of semiconductor lasers. Semiconductor lasers may be diode lasers (bipolar) or non-diode lasers such as quantum cascade (QC) lasers (unipolar). Semiconductor lasers of various types may be electrically pumped (EP) (by a DC or alternating current), or pumped in other ways, such as by optically pumping (OP) or electron beam pumping. Semiconductor lasers are used for a variety of applications and can be built with different structures and semiconductor materials, such as gallium arsenide (GaAs).
Semiconductor lasers are typically powered by applying an electrical potential difference across the active region, which causes a current to flow therein. Electrons in the active region attain high energy states as a result of the potential applied. When the electrons spontaneously drop in energy state, photons are produced (to carry away the energy lost by the transition, so as to conserve energy). Some of those photons travel in a direction perpendicular to the reflectors of the laser. As a result of the ensuing reflections, the photons can travel through the active region multiple times.
Stimulated emission occurs when an electron is in a higher energy level and a photon with an energy equal to the difference between the electron""s energy and a lower energy interacts with the electron. In this case, the photon stimulates the electron to fall into the lower energy state, thereby emitting a photon. The emitted photon will have the same energy as the original photon, and, if viewed as waves, there will be two waves emitted (from the electron""s atom) in phase with the same frequency. Thus, when the photons produced by spontaneous electron transition photons interact with other high energy state electrons, stimulated emission can occur so that two photons with identical characteristics are present. If most electrons encountered by the photons are in the high energy state, the number of photons traveling between the reflectors tends to increase, giving rise to amplification of light and thus lasing.
The use of semiconductor diode lasers for forming a source of optical energy is attractive for a number of reasons. For example, diode lasers have a relatively small volume and consume a small amount of power as compared to conventional laser devices. Further, the diode laser is a monolithic device, and does not require a combination of a resonant cavity with external mirrors and other structures to generate a coherent output laser beam.
Additionally, semiconductor lasers may be edge-emitting lasers or surface-emitting lasers (SELs). Edge-emitting semiconductor lasers output their radiation parallel to the wafer surface, while in SELs, the radiation output is perpendicular to the wafer surface. One type of SEL is the vertical-cavity surface-emitting laser (VCSEL).
The VCSEL structure usually consists of an active (optical gain) region sandwiched between two mirrors, such as distributed Bragg reflector (DBR) mirrors: a top, exit DBR, and a bottom DBR. DBRs are sometimes referred to as mirror stacks. DBRs or mirror stacks in VCSELs are typically formed of multiple pairs of layers referred to as mirror pairs. The pairs of layers are formed of a material system generally consisting of two materials having different indices of refraction and being easily lattice matched to the other portions of the VCSEL. The number of mirror pairs per stack may range from 20-40 pairs to achieve a high percentage of reflectivity, depending on the difference between the refractive indices of the layers. A larger number of mirror pairs increases the percentage of reflected light (reflectivity). The DBR mirrors of a typical VCSEL can be constructed from dielectric or semiconductor layers (or a combination of both, including metal mirror sections). The difference between the refractive indices of the layers of the mirror pairs can be higher in dielectric DBRs, generally imparting higher reflectivity to dielectric DBRs than to semiconductor DBRs for the same number of mirror pairs and overall thickness. Conversely, in a dielectric DBR, a smaller number of mirror pairs can achieve the same reflectivity as a larger number in a semiconductor DBR. However, it is sometimes necessary or desirable to use semiconductor DBRs, despite their lower reflectivity/greater thickness, to conduct current, for example (e.g., in an EP VCSEL). Semiconductor DBRs also have higher thermal (heat) conductivity than do dielectric DBRs, making them more desirable for heat-removal purposes, ceteris paribus.
When properly designed, these mirror pairs will cause a desired reflectivity at the laser wavelength. Typically in a VCSEL, the mirrors are designed so that the bottom DBR mirror (i.e. the one interposed between the substrate material and the active region) has nearly 100% reflectivity, while the top DBR mirror has a reflectivity that may be 98%-99.5% (depending on the details of the laser design). Of course, various laser structures may vary from these general properties.
VCSELs have many attractive features such as low threshold current, single longitudinal mode, and a circular output beam, among others. Additionally, because the output is perpendicular to the wafer surface, it is possible to test fabricated VCSELs on the wafer before extensive packaging is done, in contrast to edge-emitting lasers, which must be cut from the wafer and packaged to an extent necessary to test the laser.
Referring now to FIG. 1, there is shown a cross-sectional view of the layer structure of a VCSEL array 100, to illustrate heat removal from the active region of a VCSEL 110 of the array 100 (not to scale). VCSEL 110 comprises a bottom DBR 113 (part of layer 103), an active region section 114 (part of active region layer 104), and a top DBR 115 (which may also be part of a top DBR layer (not shown)). These parts of VCSEL 110 are disposed on substrate layer 102, which is mounted on submount 101. Active region 114 is pumped with some form of pumping energy (e.g., light from pump laser 118) and coherent light 119 is emitted vertically out of the top DBR 115.
During operation, the active region of the VCSEL generates heat, whether it is electrically pumped (EP) or optically pumped (OP). Heat is generated in active region 114, e.g., due to the pumping energy applied thereto, and also because the active region is not 100% efficient in converting the pumping energy to output light. For example, when electrons fall into a lower energy state without emitting a photon, heat is generated.
It is important to maintain the active region at or below a specified threshold temperature, and/or to maintain the active region at as low a temperature as possible. One reason for this is to prevent damage to the VCSEL 110. Another reason is that, generally, a semiconductor laser operates with higher gainxe2x80x94and thus more efficientlyxe2x80x94at lower temperatures. Thus, for example, at a lower active region temperature, a higher pumping energy may be applied to give rise to a higher output power laser beam than would be possible at a higher active region temperature. For example, a typical OP VCSEL, with the submount maintained at 50xc2x0 C., might stop lasing when pumped at 70 mW, and may be optimally pumped at a maximum of 50 mW. However, if the same OP VCSEL structure has its submount maintained at 20xc2x0 C., it might stop lasing when pumped at 100 mW, and may be optimally pumped at a maximum of 70 mW, therefore providing for a higher maximum output power.
Accordingly, heat must be removed from active region 114, to control the temperature of the active region. As indicated by curved arrows emanating from active region 114 in FIG. 1, the heat from active region 114 flows away from the active region, e.g. through the bottom DBR 113, through substrate 102, and into submount 101 (which may be coupled, e.g., to a TE (thermoelectric) cooler).